Optical networks are used to convey very large volumes of digital data traffic on continental and intercontinental scales, for example for Internet multimedia applications. At present optical technology provides in-fiber bit rates of the order of one Terabit per second (1012 bits per second), although the theoretical limits are much higher, and is therefore the solution of the future for exchanging high-density information, especially voice and video.
FIG. 1 shows an example of an optical communications network 2 deployed on the European continent and known as the European Optical Network (EON). Like any network, it includes a set of nodes 4, which are called switching matrices, interconnected by optical fiber links 6. The connectivity of a switching matrix 4, which expresses the number of links 6 that it connects, is typically of the order of three or four. Some switching matrices 4′ relay calls to other continents.
Two multiplexing techniques are used for packet mode optical transmission networks:                Time division multiplexing (TDM). FIG. 2a shows the basic configuration of a packet 8 conforming to this multiplexing mode, referred to hereinafter as a TDM packet. The packet has a header 10 at one end and blocks of data 12 that constitute the payload of the packet. All the elements (payload and header) of a TDM packet 8 are carried on one carrier wavelength λi suitable for the network.        
Wavelength division multiplexing (WDM) combines a plurality of independent data channels, each allocated its own carrier wavelength, on a single medium, in this instance an optical fiber, and reduces the bit rate per carrier, and therefore relaxes the physical constraints (bit rate limited modulation electronics, resistance to noise, etc.). FIG. 2b shows an example of a data packet 14 conveyed on a fiber using wavelength division multiplexing and four carrier wavelengths λ1 to λ4. Each carrier transmits respective sub-packets 8a to 8d having respective payloads 12a–12d and together forming the payload of the packet 14 conforming to this multiplexing mode, referred to hereinafter as a WDM packet. A WDM packet 14 generally includes a single header 10, which here is contained in the sub-packet 8d on the wavelength λ4. The sub-packets 8a–8d are transported in parallel in the fiber, using the principle of linear superposition.
Because the switching matrices 4 must be dedicated either to TDM packets or to WDM packets, a conventional optical network manages only one or the other of the two multiplexing modes.
Switching matrix input and output ports use multiplexing and demultiplexing means designed to work at a wavelength or at a set of wavelengths and to route packets in accordance with protocol rules imposed by the TDM mode or by the WDM mode.
There are also optical communications networks that route data in circuit mode, i.e. without dividing the data into packets. In this context, multigranularity optical network architectures have been proposed for managing wavelength division multiplexed information. The granularity expresses the basic vector that conveys data and, depending on the network and the location within the network, can be: i) the carrier wavelength, ii) a group of wavelengths, called a band, or iii) the carrier fiber. These three forms of granularity conform to a hierarchy in the sense that fiber level granularity is a physical member and transports all of the n wavelengths accepted by the network, group level granularity constitutes a subset of m wavelengths, and wavelength level granularity comprises only one of the n or m wavelengths.
The switching matrices of a multigranularity network comply with this hierarchy in providing three respective switching stages, each equipped with its own space-division switch, namely:                a first stage, disposed between the input and output fibers of a link, which extracts a bitstream from a selected fiber,        a second stage in which a demultiplexer receives the bitstream from a fiber selected by the first stage to extract a group therefrom, and        a third stage in which a demultiplexer receives a group from the second stage to extract a selected wavelength therefrom.        
These three stages are also adapted to carry out a converse series of multiplexing operations leading to a selected fiber from a wavelength or a group.
An architecture of the above kind is advantageous because it can convey different wavelengths on a common section of the network, not individually, but as a group, which lightens routing management by using only one port at a time for collective routing.
In the current state of the art, because control is asynchronous, a multigranularity architecture cannot be envisaged in other than the circuit mode. In packet mode transmission, at least some of the information takes the form of TDM packets, especially on the links 6, which makes it necessary to retain a synchronous mode.
The document WO 01/95661 discloses a method of managing data in the form of packets in an optical communications network; the method includes a step of combining a set of time division multiplex packets into a wavelength division multiplex packet to form a composite wavelength division multiplex packet.
The above document describes a node having:                a first level consisting of time division multiplex packet switching matrices, and        a second level consisting of a wavelength division multiplex packet switching matrix.        
The first and second levels are coupled by multiplexers and spectral multiplexers.
The above prior art node switches only time division multiplex packets and its core switches only wavelength division multiplex packets containing time division multiplex packets.